The overall objective is to study the mechanisms regulating immune responses in humans. T-cell subpopulations will be isolated by a new and original method (bacterial adherence) and the following immune reactions will be studied: a. Specific cytotoxicity against allogeneic and autologous lymphocytes and tumor cells. b. Responsiveness to various mitogens and bacterial cell wall components. c. The bacterial structures involved in specific attachment to lymphocyte subpopulations will be isolated, and used to: (1) separate human lymphocyte subpopulations (affinity chromatography); (2) study the possible existence of a lectin mediated recognition system. d. New bacterial strains will be screened for their ability to interact specifically with lymphocyte subpopulations.